Chinnor Campbell has all the qualities of your average six-year-old. He's a ball of energy. He's curious. And he's missing his front teeth. But two months ago, he watched a police officer kill his elder brother.

The impact of that experience became evident for a brief moment on Thursday night, when tears formed in his eyes. Joined by scores of his neighbors and family members, the boy chanted the name of his brother over and over: "I am Ramarley Graham."

He was one of about 200 Bronx residents – including numerous parents and their children – who formed a single file line facing the NYPD's 47th precinct to demand the officers responsible for the killing face legal consequences. Like many in the crowd the boy wore a button with Ramarley's picture on it. His expression was serious and he repeated his brother's name with conviction.

The boy's indignation was shared by his mother, Constance Malcolm, who stared directly at a handful of police officers gathered on top of a stairway leading to the precinct's entrance. An officer who works at the precinct, Richard Haste, shot and killed Graham in his bathroom on 2 February.

"Marley didn't die in vain," she said. "Because I'm not going to stop until we get justice. Richard Haste is not going to take my son and think we going to lay down. We're not. Not on my dead body."

The circumstances of Graham's death invoke comparisons with that of another black teenager, Trayvon Martin. Like Martin, Graham was unarmed when he was shot. And as with the Martin case, there are questions over the actions of police. But unlike Trayvon Martin, the death of Marley Graham has not led to international headlines and the sympathy of a president.

Yet at Thursday's vigil, Constance Malcolm told the crowd of her determination to seek justice. "I hope he don't have to feel the pain I'm feeling because it's not nice. It's not nice. And I don't wish it on you, but I guarantee you, we will get justice, because I'm not going to stop. A mother never lays down. I'm going to stand up for my son. He's not here but he's counting on me to stand up, and I will."

Police have said they thought Graham had a gun. The suspicion apparently prompted the officers to follow the teenager to his home and force their way inside. Originally the police said he ran into the building, fleeing from officers who told him to stop. Surveillance footage showed the claim to be untrue. The police also say Haste yelled the word "gun" before he fired a single shot into the 18-year-old's chest in front of his younger brother and his grandmother.

Graham didn't have a gun though, and Haste didn't have the training required to work in the unit to which he was assigned. He and his supervisor, Sergeant Scott Morris, have been stripped of their guns and badges. The Bronx district attorney has convened a grand jury to determine whether officers should face criminal charges.

Jeffrey Emdin, an attorney for the Grahams, said the discussion left many issues unresolved. "When we met with Commissioner Kelly we were seeking answers," Emdin told the crowd Thursday night. Edmin recited a list of questions the family wanted answered.

"Why did you say that he was running into his house? Why was it until you saw the video that you then retracted? Who told you that? Who told you there was a struggle in the house? Why are you not trying find out who is the source of that information? What are you doing to get to the truth? What is the bottom line here?

"Why are the police in that house in the first place? Why did they push the door in? How could a supervisor allow this to happen? And how could this person, who was not even qualified to be on this unit, be in the unit?"

"We still have no answers," Emdin claimed. "Nothing."

A week before Thursday's rally – the second of 18 weekly gatherings intended to commemorate each year of Graham's life – Trayvon Martin's parents traveled to New York City, where they addressed a crowd that eventually swelled into thousands and filled Manhattan's streets. Martin was one year younger than Graham. Both wore hooded sweatshirts the night they died and both were judged to be suspicious by the men who killed them.

Trayvon Martin's parents joined a rally in New York to express outrage over the killing of their son. Photograph: Siegel Jefferson/Sipa/Rex Features

Carlton Berkley, a former NYPD detective, reflected on the two cases Thursday night in front of the precinct. Berkley has worked closely with the Graham family and says he has launched his own independent investigation into the incident.

"Both cases are unique," he told the Guardian, alleging that Martin was killed by "a civilian who became judge, jury and executioner". In the case of Graham, Berkley said: "We have the police committing a crime and in the commission of their crime, they took the life of an innocent young man."

Berkley added, however, that there is a link between the incidents that motivated Bronx community members to highlight both cases: "We're here together in solidarity for both of them because the end result was that a young black man – 18, 17 years old – lost their lives behind people that call themselves law enforcement."

While Martin's death has led to international headlines and the attention of President Barack Obama, scrutiny of the Graham case has been largely confined to New York City. Regardless of the press Graham's killing receives, Patrice Gordon has committed herself to supporting his family. The reason, she says, is fear for her own child.

"I don't know why anyone else is here but I'm here because right now I'm honestly afraid to have a black son in New York City," Gordon, who is now an organizer of the weekly rallies, said. "I did not know Ramarley. I did not know his family before any of this but I felt it was my duty to answer Ramarley's call for justice for Ramarley and his family. And for those who will come, because unfortunately this will not be the last."

Gordon turned her attention to Graham's younger brother. "We have to make sure we take the responsibility for those who are to come. We have a little young man right here. We have Ramarley's little brother who unfortunately witnessed this atrocity. It's our responsibility to him, and to all those who are to come, to make sure we come out here everyday, no matter the weather."